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Top end big bore - compression not what it should be?

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Old May 24, 2022 | 08:32 AM
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What settings are you running on the DTT?
 
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Old May 24, 2022 | 10:20 AM
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Originally Posted by Adam76
I have the CV40.
The CV carb operates on vacuum so holding the throttle wide open does not open the butterfly and compression readings are not accurate. Remove the carb from the boot and test again with the same gauge; post the results.

Also post the DTT settings per Max; if settings are off, it could affect performance even if compression is close to calculated.
 
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Old May 24, 2022 | 10:45 AM
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Re: the DTT settings.
Take a minute to view the lines on the spark plug electrodes to confirm or deny timing
JMO
 
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Old May 24, 2022 | 11:21 AM
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Originally Posted by djl
The CV carb operates on vacuum so holding the throttle wide open does not open the butterfly and compression readings are not accurate. Remove the carb from the boot and test again with the same gauge; post the results.

Also post the DTT settings per Max; if settings are off, it could affect performance even if compression is close to calculated.
Good point, forgot about that one.. It's funny but my FXDX was reading a little low compared to the calcs.. I don't have any problems with power tho..
 
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Old May 24, 2022 | 06:25 PM
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Originally Posted by djl
The CV carb operates on vacuum so holding the throttle wide open does not open the butterfly and compression readings are not accurate. Remove the carb from the boot and test again with the same gauge; post the results.

Also post the DTT settings per Max; if settings are off, it could affect performance even if compression is close to calculated.
Ok thanks djl, I will remove carb altogether and re-test compression. I am also gwtting new rubber o-ring seals for the gauge, even though it's brand new, just to be sure. It's a decent quality gauge, so I'm confident that it's at least semi accurate.

Also, the DTT settings are single fire, initial on 3 and advance on 5.

Thanks
 
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Old May 24, 2022 | 06:28 PM
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Originally Posted by eighteight
Re: the DTT settings.
Take a minute to view the lines on the spark plug electrodes to confirm or deny timing
JMO
Thanks eighteight, What should they look like if the timing is correct? The color change half way up the electrode?
Thanks
 
Last edited by Adam76; May 24, 2022 at 06:30 PM.
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Old May 24, 2022 | 06:51 PM
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Originally Posted by Adam76
Ok thanks djl, I will remove carb altogether and re-test compression. I am also gwtting new rubber o-ring seals for the gauge, even though it's brand new, just to be sure. It's a decent quality gauge, so I'm confident that it's at least semi accurate.

Also, the DTT settings are single fire, initial on 3 and advance on 5.

Thanks
Probably too much timing.. Try initial 2 adv 3.. I'd expect the main jet to be anywhere in the 195 to 220 range.

 
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Old May 24, 2022 | 06:57 PM
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Originally Posted by Max Headflow
Probably too much timing.. Try initial 2 adv 3.. I'd expect the main jet to be anywhere in the 195 to 220 range.
OK thanks Max, I'll try 2 and 3. The DTT ignition system manual does say to use 2 / 2 for high compression engines.
Main jet is 195.
I have both the CV Performance velocity needle and the N8E needle.. currently running the CV needle with no shims.
 
Last edited by Adam76; May 24, 2022 at 07:41 PM.
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Old May 24, 2022 | 07:53 PM
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Originally Posted by Adam76
OK thanks Max, I'll try 2 and 3. The DTT ignition system manual said use 2 / 3 for high compression engines. Is 10:1 high compression?
Main jet is 195.
I have both the CV Performance velocity needle and the N8E needle.. currently running the CV needle with no shims.
10.5 is as far as DDT is concerned which is what you built to.. Ignore what the calculator says for CCP.. While there are many that you don't have to increase the jet size on a CV most of the guys running a stronger cam than say an andrews 26 will likely only need a 195. I've seen 220s or larger on hoter builds. On the 95 at 10.8 with a 48 degree intake close I used 2/3.. I don't have a main jet size as my 95 got a Super G. I've all bore 107 at 10.9 50 degree intake close and settings are 0 adv / curve 2. Still breaking it in but it's close..

I'll preach a little.. Give the motor what it wants not what you think it needs. I've been fooled more than once.. Try say a 210. Feels better try a 220.
 
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Old May 25, 2022 | 03:09 AM
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Originally Posted by djl
The CV carb operates on vacuum so holding the throttle wide open does not open the butterfly and compression readings are not accurate. Remove the carb from the boot and test again with the same gauge; post the results.

Also post the DTT settings per Max; if settings are off, it could affect performance even if compression is close to calculated.
Wow, thats amazing... I removed carb from manifold, put jumper cables on my battery connected up to my little Toyota's car battery (not running of course) and re-did the compression test. Now the gauge is reading 200psi front and 200 psi rear! That's not cheating is it? 😁

So I guess the 570 cams are getting enough compression. The AFR has already been very quickly evaluated on the dyno up to 3500rpm (NO full throttle or heavy load) and with my 46 pilot, 195 main and 3.5 turns out on the mixture screw I was between 12 and 14 running through the gears. The Tuner would not do a full tune until 1000 miles, so that's all the tuning info I have. DTT settings now on 2 and 2.

What else would you suggest doing to get the most from the 570s? Feels like something is really holding them back. Or maybe they're just more of a 3000rpm - 6000rpm cam? If that's the case then I'm open to suggestions for a cam change that will give me more in the 2200 - 5000 range.

Thanks

BTW, very sorry to hear what's just happened in the town near San Antonio. My prayers go out to the Texas community.
 
Last edited by Adam76; May 25, 2022 at 04:09 AM.
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